1. Field of the Invention
This invention relates to a method for adsorbing and desorbing water soluble organic compounds, and more specifically to an application method of the adsorbability of which increases when heated but decreases upon cooling.
2. Description of the Prior Art
Activated carbon has been used extensively for the recovery of valuable materials such as water soluble organic compounds, or the removal of impurities or pollutants from solutions such as aqueous solutions and for other purposes. Adsorbents making use of a nonionic styrene-divinylbenzene copolymer as their base materials have been developed in recent years. Many attempts are now under way with a view toward using them in a wide variety of fields. Since their base materials, i.e., the styrene-divinylbenzene copolymer contains an aromatic ring having extremely strong hydrophobicity, the elution of materials adsorbed on the adsorbents is effected using an acidic or alkaline solution as an eluent or eluting them with various organic polar solvents. However, such elution methods develop various problems when eluting materials susceptible to alteration in quality or nature by heat, acids, bases, organic solvents or the like, such as physiologically-active substances including antibiotics, vitamins, enzymes, proteins, steroids and the like. Even if the adsorption and elution are effected at different temperatures, it is necessary to employ such a method as effecting the adsorption at a low temperature and the desorption at a higher temperature because the adsorbability decreases when heated. Furthermore, no substantial difference in adsorbability occurs even when the temperature is changed. It is accordingly indispensable to make the elution conditions for the above elution methods severer in order to carry out the adsorption and separation with good efficiency. Thus, contamination of eluates with eluents is unavoidable. When removing contaminants or impurities from a valuable material on the other hand, it is difficult to selectively remove such unnecessary materials only from the valuable material with the result that a significant loss of the valuable material is caused since the base material contains aromatic rings of extremely high hydrophobicity and under the circumstances its hydrophobicity cannot be fully controlled. In the case of substances having water solubility but relatively-high hydrophobicity such as proteins or antibiotics, non-specific and irreversible adsorption may take place on the adsorbent. This remains as one problem causing contamination of the adsorbent.
It has recently been disclosed to use a polymer of an N-substituted acrylamide derivative as a base material for ion-exchange resin in Japanese Patent Laid-open Nos. 105,758/1981 and 96,615/1983 as well as "Biopolymers", 22, 839-847 (1983). The ion-exchange resin has been known to have such a property that it may be used even for aqueous solutions containing electrolytes at high concentrations or for various organic solvents.
On the other hand, there have been known for many years compounds having temperature-dependent hydrophobicity, in other words, cloud points when converted into organic polymers. Some attempts have already been made to utilize their temperature-dependent hydrophobicity for adsorption and separation. For example, Japanese Patent Laid-open No. 121,804/1980 contains one Example in which the adsorption of polyethyleneglycol nonylphenyl ether having a polymerization degree of 10 was conducted using an adsorbent composed of alumina and polyvinyl methyl ether carried thereon. It has recently been uncovered that the above-described phenomenon may also take place with an aqueous solution of a polymer of N-isopropylacrylamide. For example, it is disclosed on page 117-126 of the Abstracts of Research Papers Read at the 1983 Meeting of Research Institute for Polymers and Textiles, Agency of Industrial Science and Technology, Japanese Government that making use of the above-described property of the aqueous solution of the N-isopropylacrylamide polymer, the adsorption of polyethyleneglycol nonylphenyl ether having a polymerization degree of 10 was carried out using an adsorbent composed of cotton fabric and N-isopropylacrylamide graftpolymerized thereon. In each of the above disclosures, properties of the base material, i.e., the N-isopropylacrylamide polymer as an adsorbent do not appear to have been fully clarified, because the polyethyleneglycol nonylphenyl ether used as the solute contains by itself nonylphenyl groups, which are hydrophoic groups, and polyethyleneglycol groups which become hydrophobic upon heating. According to a study conducted by the present inventors, they do not seem to be satisfactory as adsorbents as demonstrated by Comparative Examples which will be given herein.
With a primary objective placing on the separation of proteins, water-soluble high-molecular compounds, and like substances, another development has also been made on a separation method in which the hydrophobicity of hydrophobic groups such as phenyl groups or alkyl groups are utilized by introducing such hydrophobic groups into polysaccarides such as agarose.
Although the above separation method does not develop such non-specific irreversible adsorption as mentioned above, upon elution of once-adsorbed protein, contamination of the eluate with an electrolyte, acid, base or the like is unavoidable. In addition, such a polysaccharide gel generally has low mechanical strength. When a solution is passeed through the polysaccharide gel in order to conduct the adsorption and emoval of contaminants, impurities or the like from the solution actually, the polysaccharide gel tends to develop such problems as pressure loss, clogging and the like. Therefore, it is not a fully satisfactory adsorbent.